The invention relates to a receiving method used in a CDMA radio system comprising, as a transmitter and a receiver, at least one subscriber terminal and a base station, which receive composite signal that has been multiplied by a spreading code into wideband, the composite signal arriving at the receiver over several paths from a plurality of transmitters, the composite signal then representing both interfering signals and desired signals and which composite signal is converted into narrowband in the receiver and detected.
The invention further relates to a receiver arranged to be used in a CDMA radio system comprising at least one subscriber terminal and a base station which are arranged to receive wideband composite signal comprising signals propagated over several paths from a plurality of transmitters, and which composite signal comprises both interfering signals and desired signals and which composite signal the receiver is arranged to convert into a narrowband signal and to detect.
In a DS-CDMA (Direct Sequence Code Division Multiple Access) the user""s narrowband data signal is modulated by a spreading code, which is more wideband than the data signal, to a relatively wide band. In systems used, bandwidths from 1.25 MHz to 50 MHz have been employed. A spreading code is conventionally formed of a long pseudo-random bit sequence. The bit rate of the spreading code is much higher than that of the data signal and in order to distinguish spreading code bits from data bits and symbols, they are called chips. Each user data symbol is multiplied by the spreading code chips. Then the narrowband data signal spreads to the frequency band used by the spreading code. Each user has his/her own spreading code. Several users transmit simultaneously on the same frequency band and the data signals are distinguished from one another in the receivers on the basis of a pseudo-random spreading code. However, the signals of different users interfere with each other in the receiver as the spreading codes are not entirely orthogonal particularly on account of a phase shift caused by a propagation delay.
Multiple access interference of the CDMA systems are reduced, for example, by using Multi-User Detection (MUD). There are several such methods and using them interference from the users cell area can best be reduced, and thus improve the capacity of the system. Known MUD solutions are based on two main types: regeneration of a wideband signal after preliminary detection or decorrelation in which a reverse matrix of a cross correlation matrix of the spreading codes is generated. In the former, interference is removed to the effect that the strongest regenerated signals are subtracted from the received wideband composite signal. Regenerating the wideband signal, however, requires a lot of calculation capacity from the receiver. In the latter, interference is reduced by multiplying a received signal vector by the reverse matrix of the cross correlation matrix of the spreading codes, the generation of which becomes more difficult as the number of users and paths increases.
An object of the present invention is to implement a solution in which it is avoided to generate a wideband signal and to reverse a cross correlation matrix of spreading codes and thus simplifying the interference cancellation of multi-user detection.
This is achieved with the method of the type set forth in the preamble characterized by subtracting a narrowband interfering signal estimate of one path of at least one transmitter from a narrowband composite signal propagated over several paths in the receiver in order to generate an interference cancelled signal.
A receiver of the invention is characterized by comprising an interference cancellation means comprising a plurality of interference cancellation primitives which are arranged to generate at least one narrowband interfering estimate signal describing interference, the interference cancellation means being arranged to subtract a narrowband interfering estimate signal from the narrowband composite signal.
Great advantages are achieved with the method of the invention. Multi-user detection and interference cancellation can be performed using simple operations without regenerating a wideband signal or reversing a cross correlation matrix. In addition, the reduction of the mutual interference of the signals simplifies power control and reduces transmission power. This, in turn, reduces interference and allows a larger capacity of the system.